Tips & Tricks: 3-2-1 Jig Theory applied to FEA Boundary Conditions
Any solid body as 6 rigid body degrees of freedom, as shown in the figure below. The goal of applying good boundary conditions in most Finite Element Analyses (FEA's) is to restrain these translations and rotations so that it can still deform in any direction without stiffening the part in any way.
If the applied constraints are just sufficient to keep the part in equilibrium and no more, this will usually provide conservative stresses and deflections. Note that most free or express versions of simulation software force over-constraint of your models, and so can be unconservative (very dangerous!)
|6 rigid body degrees of freedom|
|Jig and Fixture design is also about restraining rigid body degrees of freedom without inducing deflections into the part. The 3-2-1 theory of fixture design is illustrated below:
This restrains all 6 rigid body degrees of freedom with the minimum number of boundary conditions. Applying this jigging method to FEA results in constraining one point in all three translational freedoms, a second point in twotranslational freedoms and a third in one translational freedom.
To check that the loads are balanced and that the boundary conditions are correct, simply check that they carry no load. You can read the rest of the article here.
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